1. Technical Field
The invention relates to an electromagnetic (EM) energy projection system and more particularly to an energy collimator efficiently combining inputs from a plurality of sources with a minimum of destructive interference and projecting an energy field having a clean conic, or cylindrical, shape of high radiant power.
2. Description of the Problem
Obtaining high efficiency in the projection of radiated power or achieving high power levels in a projected beam, using transmitters of a given, limited output capacity, have been obtainable through various techniques, but doing both concurrently has proven more difficult. High efficiency can be obtained by use of a focusing reflector, such as a parabolic dish, to produce a narrow beam. However, such systems are usually inherently limited to a single input point which limits them to the use of only one transmitter. High power levels can be obtained at some loss of efficiency where the radiation fields exhibit high levels of spatial coherence. The is done with the use of phase arrays constructed from mutually coherent sources. These arrays can incorporate hundreds, or even thousands, of transmitters and can be used to produce powerful, steerable beams while suppressing side lobes. The power cost is substantial and the return from additional transmitters diminishes with expansion of the array. Arrays are effective only when the transmitters are spaced from adjacent transmitters in the array by less than one half of a wavelength of the emitted radiation, which makes them useful at radio frequencies (RF) but makes them impractical at optical frequencies (OF).
Weather radar systems provide one useful application of electromagnetic radiation projection where high power inputs are useful and the use of phase array systems widespread. In general, weather radar operates over an EM wavelength between one and 30 centimeters (i.e. microwave RF with shorter wavelengths required for smaller particle detection, such as clouds), generates pulses with a peak power of several megawatts, pulse widths of a few microseconds, a pulse repetition rate of several hundred hertz or less with a relatively narrow beam resulting from using arrays of hundreds of transmitters.
Each antenna in the linear or planar phase array is provided with its own phase shifter to produce patterns of constructive and destructive interference based on slight variations in the phase relationship of the outputs of the transmitters. The constructive interference produces a narrow, steerable beam (through phase shifting or frequency scanning) of high power. Destructive interference minimizes side lobes, though they remain present which contributes to return clutter. Each emitter in an array may be an inexpensive, low performance component, reducing cost of the system. Such arrays are almost infinitely scalable to achieve any power output desired, though at the price of diminishing returns. And such scalability comes at the cost of bulkiness. It can also be difficult to mount such a system on an aircraft due to the physical dimensions of the system. The ability to achieve the higher power outputs of multiple transmitters by coherently summing the outputs of multiple transmitters would be of great value.
In optical applications where coherence is not readily obtainable, an arc discharge lamp positioned at the focus of a parabolic reflector has been used has been favored for obtaining high intensity light beams. The ability to use a plurality of transmitters of a given, limited output capacity operating at high efficiencies, would allow the displacement of such a high maintenance cost light source.
Achieving phase coherence among distinct transmitters at optical frequencies is much more difficult and practical implementations of phase arrays at optical frequencies are not common. However, if outputs can be summed without considering coherence numerous methods exist for adding outputs from numerous sources. However, the techniques of the invention described here are not believed to have been previously applied at optical frequencies.